It is most important to center a lightguide fiber within a protective plastic coating applied thereto in order to prevent degradation of the strength thereof. Such centering also eases the fabrication of connector arrays for splicing and provides maximum abrasion protection for a given coating material.
One method of accomplishing such centering is set forth in copending patent application Ser. No. 179,594 filed in the U.S. Patent and Trademark Office on Aug. 21, 1980, assigned to the instant assignee, now abandoned, which is hereby incorporated by reference. That application discloses a technique for automatically centering a lightguide fiber in a transparent plastic coating that has an index of refraction higher than that of the fiber material. The fiber is passed through an applicator having coating material therein to coat the fiber. Orthogonal beams of laser light are directed at the coated fiber resulting in first and second forward scattered light patterns impinging on first and second screens located proximate the fiber. The patterns are monitored by a pair of CCTV cameras and the video output signals therefrom are processed to determine the distance from a center peak reference to each of two prominent outboard peaks. The distances are compared to determine the coating eccentricity and a feedback signal is developed to automatically adjust the applicator to provide a uniform, concentric, coating on the fiber.
Such a technique has been found to be most effective when the coating material has an index of refraction higher than the fiber. However, when the index of refraction is lower than that of the fiber material the amplitude of the outboard peaks substantially decreases resulting in inaccurate measurements and adjustments to the applicator. Accordingly, there is a need for a technique for automatically centering a lightguide fiber in a coating material having a lower index of refraction than that of the fiber material.